Monday, June 30, 2014

The ability to maintain
consistent bath temperature in the anodizing tank is one of the most important
factors in producing consistently high quality anodic finishes.It is widely accepted that the temperature
range for Type II and Type III anodizing is + 2o F (1oC).Other anodizing processes such as phosphoric
acid and chromic acid (Type I) anodizing don’t have quite as stringent
requirements.

While anodizers know that an
anodizing bath must be cooled, most anodizing tanks require both heating and
cooling.Only the subject of cooling is
addressed in this article.

When bath temperatures exceed
the range limits the coating characteristics are more difficult to
control.As bath temperature increases
it becomes more conductive.This allows slightly
higher current flow, which results in higher current density, and the anodic
coating is formed faster.The opposite
is true when the electrolyte is “too cold.”Wide variation in bath temperature can affect the anodic pore size,
coating thickness and coating hardness and can even affect the way a load
seals.The coating characteristics can
vary from load to load if the bath temperature is not consistent.This results in difficulty of color matching,
especially on dyed or electrolytically colored parts, but also on clear
anodized parts.

There are two common methods of
cooling the sulfuric acid anodizing electrolyte:

Placing the cooling coils in
the anodizing tank can be problematic for several reasons:

Some
coils are delicate and can be damaged by what is going on in the tank.

Coils
are susceptible to holes due to electrolytic and/or chemical corrosion
leading to possible contamination of the bath from the coolant.

Coils
made of titanium or zirconium are extremely expensive.

The
relationship between cooling coils and tank cathodes have a history of
being complicated and troublesome.

An external heat exchange
system is removed from the harsh environment of the anodizing bath.Electrolysis, bath air agitation and
accidental contact between the anodizing loads and cooling coils are not
factors with an external temperature control system.Other advantages of external systemsare:

Usually
less expensive than coils placed in the tank, depending on the size of the
tank.

Easier
to maintain because an external system is not in the tank itself.

A
plate heat exchanger is a more efficient heat transfer mechanism than
cooling coils.

Less
wear and tear overall.

Pumped,
circulating acid provides another means of bath agitation.

A
filtering system maybe easily
incorporated.

Acid
circulating pump is an easy way to pump the tank for maintenance.

Whether the anodizer chooses
cooling coils or the external heat exchanger method, regular and proper
preventive maintenance is a requirement for effective and efficient long term
performance of the cooling system.

This post was written by AAC Member Larry
Chesterfield of Anodizing Technologies Incorporated.